Various leaching processes are known for extracting precious metals (e.g., gold, platinum, silver) from various minerals. (For the purposes hereof, a “precious metal” is defined as including any of gold, platinum, and silver.) For example, in a known process to extract gold, a leaching liquor dissolves gold from an ore into a stable solution phase containing gold ligand (i.e., lixiviants). The prior art processes typically also include a step or series of steps in which the dissolved gold is collected by a recovery process, to produce solid gold metal.
Usually, the most important criteria in evaluating leaching processes are the rate of gold extraction (usually expressed as a percentage) and reagent consumption (e.g., in the prior art thiosulfate leaching processes, Na2S2O3.5H2O or (NH4)2S2O3).
In the prior art, the most commonly used lixiviant is cyanide salts. Cyanidation, however, has a number of problems. For example, cyanide is an extremely toxic chemical which, if released into the environment, may cause serious environmental damage.
Also, cyanidation does not work well with carbonaceous ores. In the prior art, the usual treatment of carbonaceous ores has involved a step of roasting the carbonaceous ores, to eliminate the carbonaceous matter before cyanidation. However, the roasting processes of the prior art are potentially harmful to the environment.
In order to address these problems, thiosulfate has been proposed as an alternative lixiviant to cyanide. Thiosulfate prevents the adsorption of gold onto the surface of carbonaceous materials, and thiosulfate is generally not particularly harmful to the environment.
One approach proposed in the prior art is to use a thiosulfate-copper-ammonia leaching system, and a typical example thereof is disclosed in U.S. Pat. No. 4,654,078 (Perez et al.) However, ammonia also can result in environmental damage, if not properly controlled. In addition, if not properly controlled in a processing facility, ammonia can cause certain occupational health problems.
There is therefore a need for an effective and stable leaching system which is significantly less harmful to the environment.